A knowledge of the polarization state of a light beam is useful in a number of applications. For example, in natural resources monitoring the details of wave structures near a shoreline may be understood more clearly by viewing an image near the shoreline in its different polarization states, as compared with the unpolarized image. In another example, a man-made object has a different polarimetric appearance than a natural object. The polarization state may therefore be used to distinguish a man-made object from a natural object in military applications such as the detection of camouflaged objects.
The polarization state of light may be specified by four observable quantities, termed its Stokes Vector. To evaluate the light according to the components of its Stokes Vector, the light beam must be sampled at least three times, with appropriate polarizers.
Polarimeters are available to perform the polarimetric analysis of the light beam that forms the image. Where the light output from the scene, and the relation between the scene and the detector, are nonvarying or vary only slowly with time, the different polarization states of the light beam may be sequentially sampled. In many situations, however, sequential sampling is insufficient, either due to the motion of the platform carrying the polarimeter or due to the rapid variation of the scene, or both. For example, if the polarimeter is in an aircraft or spacecraft that passes over the scene, or if the scene changes rapidly, the light beam constantly changes. Any delay between sequential samplings of the different polarization states makes it difficult to compare the images in their different polarization states.
Simultaneous Stokes Vector polarimeters have been developed to allow polarimetric analysis in such situations. In one approach, three or four separate polarizing imaging systems allow the simultaneous imaging of the scene in its different polarization states. While operable, such an apparatus is bulky and expensive, and the different simultaneously obtained images may be difficult to compare because of small natural variations in the registrations of the optical systems, such as small variations in the focal lengths of the three or four imaging polarimeters. In another approach, different micropolarizers may be placed over adjacent pixels of the imaging detector along with a micro lens to diffuse the incoming optical beam so that it falls equally on all detectors. While operable, the spatial resolution of the detector is significantly reduced. The light beam may be defocused in one direction, again with an associated loss of spatial resolution.
There is a need for a better approach to polarimetric analysis of time-varying light beams. The present approach fulfills this need, and further provides related advantages.